This invention relates to removable card guides for horizontal circuit boards as commonly found in “pizza-box” shaped chasses for telecommunication gear. The invention applies generally to systems of 1U to 4U in height (where “U” represents 1.75 inches). Existing systems can have different types of cards that can be replaced to offer different interfaces. However, usually it is necessary to remove the upper lid of the chassis in order to remove the cards, and it is typically impossible to change the width of a line card without removing the top lid to change the configuration of the card guides. This upgrade cannot be done in the field as it requires that the chassis be returned to the vendor, because removing the lid voids the warranty. Therefore, changing the line card width is highly service-affecting and requires significant planning. It is also important to allow for different size line cards to be inserted in the system as different combinations of interfaces require different card sizes. Furthermore, the use of different sizes of line cards reduces the amount of inventory required to build a system and minimizes the number of parts that are required to manufacture a system so the volumes can be maximized.